The meaty part of the "Working with CSS Style Sheets in an HTML5 Application" tutorial is where you start working with tools for CSS rules.
When you follow the instructions and call up the Edit CSS Rules dialog from the CSS Styles window, you may find that your CSS file isn't shown there. Instead, you can open that same dialog from the Navigator, as shown in the (soundless) YouTube movie below, which always correctly shows all the available CSS files, so that you can use the tools to write rules into those files:
Related issues: